1. Field of the Invention
The present invention relates to an exposure apparatus for printing a pattern formed on a mask onto a photosensitized substrate such a photoresist-coated wafer, and particularly suitable for use with such an exposure apparatus having an illumination optical system including a vibration source.
2. Description of the Related Art
There have been used various exposure apparatuses (including steppers) for a photolithographic process in the fabrication of semiconductor devices, liquid crystal displays or the like, in which a pattern on a reticle (or mask) is transferred by exposure onto each shot area on a photoresist-coated wafer (or glass plate). In a one-shot exposure type of exposure apparatus such as a stepper, in order to print a pattern formed on a reticle onto each shot area on a wafer, the reticle and the wafer have to be held almost completely stationary. For this purpose, such an exposure apparatus has its platen (or its surface block) mounted on a floor through vibration isolators so as to substantially prevent any harmful vibrations from transmitting from the floor to the platen and to the portion of the exposure apparatus on the platen (i.e, the main portion of the exposure apparatus).
Recently, exposure apparatuses using the scanning projection exposure technique (such as the step-and-scan type of exposure apparatus) have drawn engineers"" attention because they enable transferring a significantly larger pattern formed on a reticle onto a wafer, without the need for a larger size of projection optical system. In the scanning projection exposure apparatus, the reticle is moved in a direction perpendicular to the optical axis of the projection optical system for scanning, while the wafer is moved in synchronism with the reticle and in a direction corresponding to the direction of the movement of the reticle for scanning, with the ratio between the velocities of the reticle and the wafer being equal to the demagnification ratio of the projection optical system, so that the pattern on the reticle is serially transferred by exposure onto the wafer. In the scanning projection exposure apparatus with such arrangement, vibration isolators have to be used to reject any harmful vibrations which would otherwise transmitted from the floor, in order to ensure that the reticle and the wafer may be moved for scanning at constant velocities with stability during the scanning exposure operation.
FIG. 5 is a schematic representation, partially cutaway, showing a typical stepper type of exposure apparatus. As shown in FIG. 5, the exposure apparatus has an illumination optical system 37 comprising an illumination system lens barrel 42 and other components mounted on the illumination system lens barrel 42, the components including a light source 39 such as a mercury-vapor lamp, a relay lens 40A, a reticle blind (field stop) 41, condenser lens 40B, and a cooling fan 43 with a casing 43A covering the cooling fan 43 for introducing cooled air through a cooling-air duct 38 into the illumination system lens barrel 42. The illumination optical system 37 comprises further components including an optical integrator, but they are not shown in the figure for simplicity. Below the illumination optical system 37, there are disposed, from upper to lower positions in the following order, a reticle stage 27 for carrying a reticle, a projection optical system 25, and a wafer stage 20 for carrying a wafer 22.
The projection optical system 25 is directly mounted on a first column 124, the reticle stage 27 is mounted to the first column 124 through a second column 126, and the illumination system lens barrel 42 confining the components of the illumination optical system is mounted to the first column 124 through a third column 136. The first column 124, in turn, is fixedly mounted on a platen 106 (or a surface block 106). The wafer stage 20 carrying the wafer 22 is also mounted on the platen 106. The platen 106, the wafer stage 20, the first column 124, the projection optical system 25, the second column 126 and the reticle stage 27 together compose the main portion of the exposure apparatus. Accordingly, the illumination optical system 25 is mounted to the main portion of the exposure apparatus through the third column 136.
There are disposed on the floor 1 three or four base plates (of which FIG. 5 shows only two base plates 102A and 102B), on which three or four vibration isolators (of which FIG. 5 shows only two vibration isolators 111A and 111B) are mounted one on each base plate, for supporting the platen 106. The exposure apparatus further includes various alignment systems for establishing the alignment between the reticle 28 and the wafer 9, which alignment systems are, however, not shown in the figure.
In this conventional arrangement, the illumination optical system 37 is directly connected to the main portion of the exposure apparatus, that is, an integral structure is used. This arrangement, however, generates various sorts of vibrations, one of which is the vibration produced by the cooling fan 43 for removing the heat generated by the light source 39 and transmitted to the illumination optical system 37, and the others of which are the vibrations produced by a fan in an air conditioning system and/or by a pump for a water-cooling system. Therefore, this arrangement suffers from a drawback that such vibrations will be transferred to the stage system such as the wafer stage 20, so that the alignment accuracy of the exposure apparatus and the positioning accuracy for positioning the stage are deteriorated thereby.
It is an object of the present invention to provide an exposure apparatus capable of positioning a wafer with precision and without being influenced by any vibrations produced for example by a cooling fan for cooling an exposure light source of an illumination optical system, by virtue of an arrangement wherein the vibrations are transferred to an independent column separate from the main portion of the exposure apparatus, and thereby the vibrations are prevented from being transferred to an alignment system and a stage system provided on the main portion of the exposure apparatus.
It is another object of the present invention to provide an exposure apparatus capable of positioning a wafer with further precision, by virtue of an arrangement wherein a misalignment detection mechanism is provided between the illumination optical system and the exposure machine main portion (which are separated from each other) for detecting any misalignment between them, and the misalignment detection mechanism is used to detect not only the vibrations produced for example by the cooling fan but also any misalignment which may be caused by the vibrations as produced by the movement of a reticle stage and/or a wafer stage, and to cancel out any of such vibrations by correction.
In order to achieve the above and other objects, according to the present invention, as shown in FIG. 1, there is provided an exposure apparatus for transferring a pattern formed on a mask (28) onto a photosensitized substrate (22), comprising: an illumination optical system (37) for illuminating the mask (28) with an exposure illumination beam (IL); an exposure machine main portion (29) serving to hold the mask (28) and the photosensitized substrate (22); and first and second support structures on which the illumination optical system (37) and the exposure machine main portion (29) are mounted, respectively, the first and second support structures being separate from each other.
In this arrangement, it may be preferable that: the second support structure on which the exposure machine main portion (29) is mounted comprises a vibration isolation structure (3A, 3B, 4A, 4B, 5A, 5B, 6, 7A, 7B, 31A, 31B, 32A, 32B, 35A, 35B) capable of six-degree-of-freedom position and orientation control; the exposure apparatus further comprises a position detection mechanism (44A, 44B) for detecting any misalignment between the illumination optical system (37) and the exposure machine main portion (29); and the position and orientation of the vibration isolation structure are controlled based on detected values from the position detection mechanism (44A, 44B).
In addition, the illumination optical system (37) may preferably further comprise a field stop (41) for limiting an illumination area on the mask (28) and the field stop (41) is fixedly mounted on the exposure machine main portion (29).
According to the exposure apparatus of the present invention with the above arrangement, because the illumination optical system (37) and the exposure machine main portion (29) are mounted on the separate first and second support structures, any vibrations and shakings of the illumination optical system (37) which may be produced for example by the cooling fan and other components which are generally mounted on the illumination optical system (37) as integral components will not influence upon the alignment accuracy between the mask (28) and the photo-sensitized substrate (22), so that the alignment is achieved with precision.
In the case where the second support structure on which the exposure machine main portion (29) is mounted comprises a vibration isolation structure (3A, 3B, 4A, 4B, 5A, 5B, 6, 7A, 7B, 31A, 31B, 32A, 32B, 35A, 35B) capable of six-degree-of-freedom position and orientation control, where the exposure apparatus further comprises a position detection mechanism (44A, 44B) for detecting any misalignment between the illumination optical system (37) and the exposure machine main portion (29), and where the position and orientation of the vibration isolation structure are controlled based on detected values from the position detection mechanism (44A, 44B), then, the position detection means (44A, 44B) can be used to measure any misalignment between the illumination optical system (37) and the exposure machine main portion (29), and the vibration isolation structure having six-degree-of-freedom can be used to control the position and orientation of the exposure machine main portion (29) with precision so as to correct any misalignment between the exposure machine main portion (29) and the illumination optical system (37).
In the case where the illumination optical system (37) further comprises a field stop (41) for limiting an illumination area on the mask (28) and the field stop (41) is fixedly mounted on the exposure machine main portion (29), the relative position of the field stop (41) and the exposure machine main portion (29) remains unchanged even when a misalignment occur between the illumination optical system (37) and the exposure machine main portion (29), so that the illumination area can be maintained stationary.